A moving image encoder (an encoding apparatus) which generates coded data by encoding a moving image and a moving image decoder (a decoding apparatus) which generates a decoded image by decoding the coded data are used to efficiently transmit or record a moving image. A specific moving image encoding method is exemplified by methods employed in H.264/MPEG-4.AVC (Non Patent Literature 1) and KTA software which is CODEC for joint development in VCEG (Video Coding Expert Group).
Further, according to such an encoding method, an image (a picture) constituting a moving image is managed by a hierarchical structure which is composed of (i) a slice obtained by dividing the image, (ii) a macroblock obtained by dividing the slice, and (iii) a block obtained by dividing the macroblock, and the image is normally encoded for each block.
Further, according to such an encoding method, normally, a predicted image is generated in accordance with a locally decoded image obtained by encoding/decoding an input image, and difference data between the predicted image and the input image is encoded. A known method for generating a predicted image is exemplified by methods called inter prediction and intra prediction.
According to the intra prediction, predicted images in an identical frame are sequentially generated in accordance with a locally decoded image in the identical frame. Specifically, according to the intra prediction, normally, any one of prediction directions included in a predetermined prediction direction (prediction mode) group is selected for each prediction unit (e.g., block), and a predicted pixel value in a prediction target region is generated by extrapolating a pixel value of a reference pixel of a locally decoded image in the selected prediction direction. Meanwhile, according to the inter prediction, a predicted image in a prediction target frame is generated for each prediction unit (e.g., block) by applying motion compensation using a motion vector to a reference image in a reference frame (decoded image) which has been entirely decoded.
Each of Non Patent Literature 2 and Non Patent Literature 3 discloses an adaptive loop filter (hereinafter simply referred to as an “adaptive filter”) which divides a (locally) decoded image into a plurality of regions and carries out a filter process while changing a filter coefficient group for each of the plurality of regions in accordance with a magnitude of activity indicative of local disorder of the (locally) decoded image in the each of the plurality of regions. Note here that the filter coefficient group for the each of the plurality of regions is determined by an encoding apparatus so that a difference between a filtered image in the each of the plurality of regions and an encoding target image is minimized.
An encoding apparatus and a decoding apparatus each including such an adaptive filter allows an increase in prediction accuracy and an improvement in coding efficiency by generating a predicted image with reference to a filtered decoded image which is obtained by causing the adaptive filter to carry out a filter process with respect to a (locally) decoded image.